1. Field of the Invention
This invention relates to treating a well which penetrates a subterranean formation and more particularly to introduction of a fluid medium into a borehole in the subterranean formation to form a fluid flow blocking means.
2. Description of the Prior Art
During the drilling of an oil or gas well it is sometimes necessary to introduce fluids into the well to kill the well. This situation may occur if a formation fluid influx cannot be contained by closing the blowout preventers or by circulating the high density mud. For example, when an unusually high pressure formation is encountered, it may be necessary to employ drilling mud at such high weight that a formation above the high pressure zone is fractured. This fractured zone then becomes a "lost zone" into which mud flows at such a high rate that "lost circulation" occurs. The lost circulation may be so severe that it ultimately becomes impossible to maintain a column of mud above the high pressure zone sufficient to impart the necessary hydrostatic head to offset the high pressures in the high pressure zone. As this occurs, the well becomes increasingly susceptible to blowout into the lost zone or to the surface.
To overcome these wellbore problems, it has been suggested to plug cracks, crevices and fissures in the leaky portion of the formation and if loss of circulation becomes too severe, to plug the wellbore.
Various techniques have been suggested for preventing lost circulation. Generally lost circulation is controlled by reducing the mud density or by adding various materials to the drilling mud. These materials have frequently been of a character intended to form a type of mat intended to plug the leaky formation and thus retard or prevent the escape of circulation fluid. Examples of lost circulation materials include a broad range of fibrous, flaky, or granular materials such as cellophane, waste paper, walnut hulls, or cotton lint. It has also been suggested to introduce into the well a hydratable material such as bentonite clay which gels or stiffens into a semi-solid mass upon mixing with water.
In some cases methods and compositions heretofore used for preventing lost circulation have proven successful. However, under varying circumstances these methods have been inadequate to prevent loss of drilling fluids.
Sometimes it is necessary to plug the wellbore in order to prevent loss of drilling fluids or influx of formation fluids into the borehole. Various well control techniques have been proposed for plugging wells. One suggested technique involves placing a high density barite slurry (barium sulfate) in the annulus adjacent the high pressure zone to provide the extra hydrostatic head needed to stop or prevent formation fluid influx. If the barite slurry remains deflocculated after placement at the bottom of the well, the barite settles uniformly to form a hard plug. One problem with using barite to form a plug is that the barite's ability to plug varies greatly depending upon the formation temperature, the operating conditions, and the quality of barite used. For example, it is sometimes difficult to plug a well in the presence of a significant flow movement in the wellbore. If the fluid influx is not killed immediately by the hydrostatic head of the barite slurry, the settling barite will usually not stop the unwanted flow.
Another proposed well control technique is to inject a highly viscous fluid into the wellbore. A well known water-sensitive fluid for plugging a wellbore is a slurry of bentonite dispersed in diesel oil, a fluid commonly called "gunk." The gunk is placed in the well by circulating the slurry with spacers before and after to segregate the material from the water-based drilling fluid in the drill pipe. Upon mixing with water and mud in the annulus, the bentonite hydrates to form an extremely thick paste. The highly viscous slurry slows the formation flow and, if sufficient slurry enters the annulus, provides a complete seal. One problem with the use of gunk is that the material is highly dependent upon the amount of water that is mixed with it. It is difficult for gunk to form an adequate plug if it is exposed to either too much or too little water in the wellbore. The techniques using gunk are therefore susceptible to erratic field performance.
There is a continuing need for an improved method of forming a plug in cracks and crevices of leaky formations to prevent lost circulation and forming a plug in a wellbore which adequately seals the bottom of the well against high pressure gradients.